Linear motor relay

ABSTRACT

A linear motor relay is described for use in controlling switches and the like. The relay includes an electromagnetic drive in the form of a generally planar movable element and a generally planar stationary element. In one embodiment, a printed circuit conductor pattern is located on one element for coaction with longitudinal segments of a magnetic field produced by strips of unipolar permanently magnetic material located on the other element. In another embodiment an electromagnetic drive is obtained with coacting planar windings located on both the movable and stationary elements. A small layered structure may be formed capable of high output force for controlling a plurality of switches. A latching feature is provided to maintain the movable element in the position to which it has been driven by the electromagnetic drive. Stops are employed to retain a group of conductor turns in coacting relationship with a longitudinal segment of the magnetic field for bidirectional control upon a reversal of current flow.

United States Patent 1 Fulenwider 1 LINEAR MOTOR RELAY [75] Inventor: John E. Fulenwider, Concord, Mass.

[73] Assignee: GTE Laboratories Incorporated,

Waltham, Mass.

22 Filed: Sept. 25, 1972 21 App1.No :291,959

[52] US. Cl. 335/126, 310/12 [51] Int. Cl. H01h 67/04 [58] Field of Search 310/12, l3, l4;

Primary Examiner-Harold Broome Attorney-Irving M. Kriegsman [451 Dec. 4, 1973 [57] ABSTRACT A linear motor relay is described for use in controlling switches and the like. The relay includes an electromagnetic drive in the form of a generally planar movable element and a generally planar stationary element. In one embodiment, a printed circuit conductor pattern is located on one element for coaction with longitudinal segments of a magnetic field produced by strips of unipolar permanently magnetic material located on the other element. In another embodiment an electromagnetic drive is obtained with coacting planar windings located on both the movable and stationary elements. A small layered structure may be formed capable of high output force for controlling a plurality of switches. A latching feature is provided to maintain the movable element in the position to which it has been driven by the electromagnetic drive. Stops Y are employed to retain a group of conductor turns in coacting relationship with a longitudinal segment of the magnetic field for bidirectional control upon a reversal of current flow.

15 Claims, 8 Drawing Figures LINEAR MOTOR RELAY This invention relates to an electromagnetic actuator. More specifically, this invention relates to a latching linear motor relay for use in telephone switching applications and the like.

BACKGROUND OF THE INVENTION Linear induction machines are well-known in the art; see for example a recent description of such devices in an article entitled Linear Induction Machines by Mia chel Poloujadoff in the IEEE Spectrum of February, 1971 on page 72. The linear induction machine is used for tractive purposes to, for instance, propel a vehicle along a path. The windings employed in such machine are stretched out along the travel direction and a special magnetic field is created to cause direct. translation motion of the vehicle.

SUMMARY OF THE INVENTION In one embodiment of a linear motor relay in accordance with the invention, a generally planar stationary element and a correspondingly shaped generally planar movable element are mounted parallel with one another. The movable element is mounted for movement along an operating direction but is limited in its motion for bidirectional control. The magnetic field is generated in the form of spacedsegments which span the gap between the stationary and movable elements for coaction with parallel like spaced groups of turns of an electrical winding mounted on one of the elements. The magnetic field segments have the shape of longitudinal strips each of which is parallel with a group of turns. The orientations of the magnetic field segments and the turns of the electrical winding are selected parallel with the surfaces of the planar elements and transverse to the direction of operation. Hence, upon the flow of current in the electrical winding a force is exerted on the movable element to advance it along the operating direction. The motion of the movable element is limited to maintain each group of turns in coacting relationship with one segment of the magnetic field. In this manner bidirectional control is obtained by controlling the direction of current flow through the electrical winding.

The linear motor relay structure in accordance with the invention is advantageously employed in a compact multilayered structure wherein a plurality of movable and stationary elements are interleaved with one another to increase the electromagnetic force. With such multilayered structure, a plurality of switches may be controlled or other heavier devices operated and higher operating speeds are obtained. As described in a second embodiment in. accordance with the invention, a multilayered structure is provided wherein the electromagnetic drive is formed with interacting planar windings locatedrespectively on the movable and stationary elements. Interleaved unmagnetized highly permeable segments placed between groups of windings enhances the force and speed of the relay.

The linear motor relay in accordance with the invention advantageously employs a printed circuit to form the electrical winding to thus provide a light-weight, economic and simple structure which is capable of providing a substantial force with a largedisplacement'of the movable element. The relay may be used to control a plurality of switches or some other output device such as the shutter for alight beam or a valve'which controls fluid discharges from a small pipe.

The linear motor relay as described with reference to a particular embodiment, is provided with a latching capability. The latch feature is obtained with a toggle device which supports the movable element and has two stable positions along the operating direction. Hence, when current is applied and the movable element moved to a new position, the element remains at this new position when current is removed.

It is, therefore, an object of the invention to provide a linear motor relay with a light-weight, economic and simple structure. his a further object of the invention to provide a latching linear motor relay switch for use in telephone switching circuits and the like.

DESCRIPTION OF DRAWINGS These and other objects and advantages of a linear motor relay in accordance with the invention may be understood from the following description of several embodiments described in conjunction with the drawings wherein FIG. 1 is a perspective view of a linear motor relay switch in accordance with the invention;

FIG; 2v isa plan .view of the linear motor relay switch of FIG. 1 illustrating the relationship between the electrical winding and magnetic field producing strips;

FIG. 3 is a perspective partially broken away view of a multilayered structure for a linear motor relay in accordance with the invention;

FIG. 4 is a section view of the linear motor relay shown in FIG. 3 and taken along the line 44;

FIG. 5 is a side view in elevation taken along the direction of movement of the movable element of a multilayered linear motor relay switch in accordance with the invention;

FIG. 6- is a section view taken along the line 6-6 of the linear motor relay switch shown in FIG. 5;

FIG. 7 is a section view of still another embodiment of a linear motor relay in accordance with the invention; and

FIG. 8 is a partial section view of the linear motor relay of FIG. 7 with elements thereof in a different operational position.

DETAILED DESCRIPTION OF EMBODIMENTS With reference to FIG. 1, a linear motor relay switch 10 isshown. The switch is formed of a generally planar stationary element in the form of a'plate 12 over which a generally planar movable element in the form of a plate 14 is mounted for movement in the operational direction indicated by double headed arrow 16. The movable element 14 is supported by four bell crank springs 18 each of which has two stable positions (20 as shown in full'and 22 in dotted line) on opposite sides of an over-center" position. Stationary element 12 supports a stationary switch contact 24 located for engagement by a movable contact 26 mounted on movable element 14. A flexible lead 28 is coupled to the movable contact 26 to allow the latter to freely move with the movable element 14.

The bell cranks springs 18 are mounted for pivot motion in sockets 30 in the stationary plate 12 and pivotally engage recesses 32 in the movable plate 14. Hence, upon the operational movement of the plate 14, the bell cranks l8'are pivoted between their stable positions 20 and 22 and slightly bend as they are carried past their over-center positions. The motion of movable element thus proceeds along the direction of arrow 16 from the solid line indicated position, where edge 34 seats against a pair of stops 36-36, to the dotted line position where the other edge 38 seats against stops 40-40.

Actuation of linear motor relay is obtained with an electromagnetic device formed by an electrical winding 42 and strips 44 of permanently magnetic material. Electrical winding 42 is mounted to the underside surface of movable plate 14 facing the magnetic strips 44 which are mounted on surface 46 of stationary plate 12.

As illustrated in FIG. 2, electrical winding 42 is formed of groups 48 of turns arranged in a wavewound three cycle manner. The electrical winding is preferably formed as a printed circuit on a flat insulative board. Below each group 48 of turns is a strip 44 of magnetic material which has been polarized with a single magnetic pole characteristic to provide alternate strips of North and South magnetic poles as shown.

Magnetic strips 44 are formed of commercially available magnetic ceramic material wherein the magnetization is frozen to impart a unipolar, North or South, characteristic to the strip. A useable magnetic material is know as Magnyl Magnetic Strip obtained from The Edmund Scientific Company in New Jersey under its catalogue item 60206. The strip is provided with an adhesive backing for ease of mounting.

Strips 44 are spaced along the direction indicated by arrow 16 with alternating North and South magnetic poles as shown in FIG. 2. The resulting magnetic field extends upwardly from each strip to intersect a group of turns 48 and coact with the current flowing through the conductors in the turns. Stops 36-36 and 40-40 are so spaced that each group of turns 48 remains in coacting relationship with one magnetic strip 44. Hence, when current through electrical winding 42 is reversed, the movable element may move to seat against stops 40-40 without overshoot. The width, W, of a magnetic strip 44 is thus selected wide enough to maintain coacting relationship with a group of turns 48 throughout the displacement range of the movable element 14.

The movable element 14, or armature winding, is located as shown in FIG. 2 with a current in the direction as shown by arrow 50. If thereafter a reverse current is applied, the interaction of the current with the magnetic flux from the magnetic strips 44 results in a net force in the plane of the electrical winding 42 and perpendicular to the long dimension of strips 44 and turns 48. As a result, the movable element 14 moves in a linear translatory motion to seat against stops 40-40.

Bell crank springs 18 support the movable element 14 and their over-center or toggle action imparts two stable positions to the armature. Hence, the relay 10 exhibits a latching capability in both directions upon removal of current.

The relay structure shown in FIGS. 1 and 2 lends itself advantageously to produce larger forces by forming a multilayered construction such as illustrated in FIGS. 3 through 6. The relay structure in FIGS. 3 and 4 show an armature 60 in the form of an insulative plate- 62 with a printed circuit conductor pattern to form an electrical motor winding 64. I 1

The armature 60 is mounted between two stationary layers 68 and 70 of magnetic field producing strips 44. The winding 64 is formed of distributed groups of turns 72.1-72.4 located between pairs of opposite polarity magnetic strips 44 as illustrated in FIG. 4. Each group of turns is made of a printed conductor pattern on opposite sides of insulator plate 62 with the conductors 72 placed over spaces 76 between adjacent conductors on the opposite side of plate 62. In this manner maximum interaction between currents flowing through conductors 72 and the magnetic field from strips is obtained. The location of the magnetic strips 44 on both sides of armature 60 produces a magnetic field which is essentially transverse to both the plane of the conductor pattern 64 and the direction of operation indicated by arrow 16.

The multilayered relay structure of FIGS. 3 and 4 provides a substantially greater force with a faster actuating capability than the embodiment of FIGS. 1 and 2.

Higher forces may be generated by employing a multilayered relay structure 79 such as shown in FIGS. 5 and 6. A plurality of movable plates 60.1-60.3 are connected together with brackets 80-80 to form a movable layered armature 82. The movable elements or plates 60 are interleaved between stationary plates 84 carrying embedded magnetic strips 44 facing conductor patterns 64 such as shown in FIG. 3. The polarity of the magnetic strips 44 are arranged as shown in FIG. 6 with the conductive pattern oriented so that the forces produced by the current all add to a substantial net force. The armature 82 is shown with plates 62 seated against stops 86 at one end of the translation path and stops 88 are provided when the armature is moved in the opposite direction. The relay 79 is shown provided with a plurality of switch contacts 24, 26, though other devices may be controlled as desired.

FIG. 7 shows a section of a multilayered linear motor relay 88 formed of a linearly movable armature plate 90 carrying planar groups of conductor turns 92-92 on both opposite surfaces 94-94. The turns may be arranged in a desired multiple of waves, two being shown but more may be accommodated.

The armature plate 90 is located between stator plates 96-96, each of which is provided with an electrical planar winding formed of groups of turns 98-98. The planar armature 90 and stator plates 96-96 are held together with linear bearing structures 100-100 which provide both a toggle action and an accurate support of the armature. A switch 102 formed of a stationary contact 104 and an armature supported movable element 106 is controlled by relay 88. Strips 107 of unmagnetized highly permeable magnetic material are placed between each pair of groups of turns to control magnetic leakage fields.

The bearing structures 100-100 are each formed of stationary linear races 108-108 and movable linear races 110-110 respectively mounted opposite each other with a bearing ball 112 between each pair of races. A pair of clips 114-114 retain the linear bearing structures and hold the entire assembly together.

The movable armature races 110-110 are each provided with curved resilient race surfaces 116-116 to reduce the space between pairs of races. The resiliency of surfaces 116-116' enables the balls 112 to move from one end of a race to the other with a toggle action wherein the end ball positions are stable. The curvature of resilient race surfaces 116-116 is slight, being of the order of 10 times the radius of balls 112. The height of a surface is small to reduce the force needed to overcome the toggle action. The bearing structures 100-l00 enable a very small gap a to be maintained between the armature plate winding 92 and stator winding 96. With a clearance gap d of 0.001 of an inch a force of 40 grams can be produced with an entire relay structure size of about 0.57 inch X 0.38 inch X 0.06 inch.

The operation of the linear motor relay of FIG. 7 is initiated by passing current through the windings as shown for the position of the armature plate 90. Thus, points represent current flowing out of the plane of the figure and crosses represent current flowing into the plane. The resulting vector forces between groups of turns (e.g. 92.198.1 and 92.198.l') is as shown by arrows l181l8'. The net force is balanced in the operational direction indicated by arrow 16.

When the toggle action of bearing structures 100-l00 have been overcome, the armature plate 90 comes to rest when bearing balls 112 seat against the other ends of the races. This latter end position corresponds to the placement of the armature groups of turns 92 near the next adjacent groups of turns on stator plates 96 as shown in FIG. 8.

A reversal of either the current through the armature winding 92 or stator winding 98, as shown in FIG. 8, will then impose a returning force, as indicated by arrow 16, on armature plate 90 causing it to close switch 102.

Having thus described several embodiments for a linear motor relay, its advantages may be appreciated. The gaps between the electrical windings and magnetic strips may be made quite small, of the order of several thousandths of an inch for a high magnetic field intensity. The size of the layered linear motor relay may be made quite small to fit into an integrated circuit package. ln one linear motor relay built according to the invention as shown in FIG. 1, the strips of magnetic material were about 0.057 inches wide and 1.2 inches long and are placed at centers of 0.175 inches. The relay is particularly useful in connection with the control of switches though it may be used for other applications. Although the embodiments show the movable elements carrying the electrical windings, in some applications the movable element may supportthe magnetic strips and the stationary elements support the electrical winding.

What is claimed is:

1. A linear motor relay comprising an electromagnetic layered structure formed of a generally planar stationary element and a generally planar movable element spaced from and mounted parallel with the stationary element for movement along an operating direction, means mounted to the stationary and movable elements for producing an electromagnetically induced force on the movable element, said force acting parallel with the operating direction, said means including a planar electrical winding mounted on one element and arranged in groups of turns which are oriented generally transverse to the operating direction to alternately carry currents in opposite directions, and strips of magnetic materials distributed on the other element with a strip opposite to and parallel with a group of turns for magnetic coupling therewith and coaction with currents carried thereby, said magnetic strips being alternately formed of unipolar opposite magnetic polarity to provide a magnetic field for coaction with the electrical winding to provide said force on the movable element, 7

means for limiting the motion of the movable element along the operating direction to maintain each magnetic strip in coacting relationship with its oppositely disposed group of turns for bidirectional control of the movable element, and

means for latching the movable element to maintain the latter in a displaced position upon the removal of current through the electrical winding.

2. The linear motor relay as claimed in claim 1 wherein said latching means includes toggle supports mounted on the stationary element to support the movable element, said toggle supports having each a pair of stable positions spaced along the operating direction with the force being sufficient to move the movable element between the stable positions of the toggle supports.

3. The linear motor relay as claimed in claim 1 wherein said stationary element supports the magnetic strips of material and the movable element supports the electrical winding.

4. The linear motor relay as claim in claim 3 and further including a second generally planar stationary element mounted over the movable element to place the latter element in sandwich relationship between the stationary elements, said second stationary element being provided with magnetic strips which are aligned with the magnetic strips on the other stationary elementand are of opposite magnetic polarity to provide a magnetic field which extends essentially transverse to the movable element between aligned magnetic strips.

5. The linear motor relay as claimed in claim 4 wherein said electrical winding is located on opposite planar surfaces as the movable element with the turns on one surface overlying spaces between turns on the other surface for enhanced interaction with the magnetic field between aligned magnetic strips.

6. The linear motor relay as claimed in claim 1 including a plurality of movable elements coupled to one another to form a multilayered armature structure and a plurality of stationary elements coupled to one another, said plurality of elements being interleaved to form alternate layers of movable and stationary elements arranged in an electromagnetic drive structure capable of producing a substantial force upon the application of current to the electrical winding.

7. A linear motor relay switch comprising a plurality of switches each having a stationary contact and a movable contact,

a multi-layered structure formed of interleaved parallel mounted sets of stationary and movable plates,

the set of movable plates being connected together to form a unified armature which moves along a 7 linear direction, said unified armature being coupled to the movable contacts for controlling the opening and closing of the switches,

meansmounted to one set of plates for producing a an electrical winding mounted to the other set of plates, said electrical winding being formed of groups of turns parallel to and in the path of the longitudinal segments of magnetic field and oriented to produce a common force on the unified armature when a current is passed through the electrical winding, and

means for limiting the motion of the unified armature to maintain each group of turns in coacting relationship with a longitudinal segment of magnetic field for bidirectional control of the unified armature to open and close said switches.

8. The linear motor relay switch as claimed in claim 7 wherein the magnetic field producing means includes strips of magnetic material of opposite magnetic polar-- ity, said strips being mounted on the surfaces of said one set of plates and transverse to the linear direction, with magnetic strips on adjacent plates of the one set being of opposite polarity to establish said strip of magnetic field between them and through an interleaved plate of the other set.

9. The linear motor relay switch as claim in claim 8 wherein the strips of magnetic material are mounted on the stationary plates and the electrical winding is mounted on the movable plates.

10. The linear motor relay switch as claim in claim 9 wherein the groups of turns are distributed on opposite surfaces of a movable plate and are located to overlay spaces between adjacent turns on one surface.

11. A linear motor relay comprising an electromagnetic structure formed of a generally planar stationary element and a generally planar movable element spaced from and mounted parallel with the stationary element for movement along an operating direction,

both said stationary and movable elements being provided with planar windings formed of groups of turns oriented transverse to the operating direction and spaced uniformly from one another along the operating direction,

with each group of turns on the movable element being in electromagnetic coacting relationship with corresponding groups of turns on the stationary element to produce a force on the movable element acting parallel to the operating direction upon the selective flow of current through the respective windings of the stationary and movable elements, and

means for limiting the motion of the movable element along the operating direction to maintain each group of turns on the movable element in electromagnetic coacting relationship with its corresponding groups of turns on the stationary element for bidirectional control of the movable element.

12. The linear motor relay as claimed in claim 11 and further including a multilayered structure formed of at least a pair of stationary planar elements and a movable planar element mounted between the pair of stationary elements.

13. The linear motor relay as claimed in claim 12 and further including groups of turns located on opposite sides of the planar movable element with the groups of turns on each side mounted for electromagnetic coaction with a group of turns on the stationary elements.

14. The linear motor relay as claimed in claim 13 and further including parallel strips of unmagnetized highly permeable material, said strips being located between groups of turns on both the movable and stationary elements.

15. The linear motor relay as claimed in claim 11 wherein said motion limiting means further includes a linear ball bearing structure mounted to the movable and stationary elements, said structure having outer linear races mounted to the stationary element and inner linear races mounted to the movable element and bearing balls mounted for movement between opposed pairs of outer and inner linear races, said races being aligned along the operating direction with one of the races in each pair having a bulge sized to define stable bearing ball positions corresponding with the limits on the movement of the movable element. 

1. A linear motor relay comprising an electromagnetic layered structure formed of a generally planar stationary element and a generally planar movable element spaced from and mounted parallel with the stationary element for movement along an operating direction, means mounted to the stationary and movable elements for producing an electromagnetically induced force on the movable element, said force acting parallel with the operating direction, said means including a planar electrical winding mounted on one element and arranged in groups of turns which are oriented generally transverse to the operating direction to alternately carry currents in opposite directions, and strips of magnetic materials distributed on the other element with a strip opposite to and parallel with a group of turns for magnetic coupling therewith and coaction with currents carried thereby, said magnetic strips being alternately formed of unipolar opposite magnetic polarity to provide a magnetic field for coaction with the electrical winding to provide said force on the movable element, means for limiting the motion of the movable element along the operating direction to maintain each magnetic strip in coacting relationship with its oppositely disposed group of turns for bidirectional control of the movable element, and means for latching the movable element to maintain the latter in a displaced position upon the removal of current through the electrical winding.
 2. The linear motor relay as claimed in claim 1 wherein said latching means includes toggle supports mounted on the stationary element to support the movable element, said toggle supports having each a pair of stable positions spaced along the operating direction with the force being sufficient to move the movable element between the stable positions of the toggle supports.
 3. The linear motor relay as claimed in claim 1 wherein said stationary element supports the magnetic strips of material and the movable element supports the electrical winding.
 4. The linear motor relay as claim in claim 3 and further including a second generally planar stationary element mounted over the movable element to place the latter element in sandwich relationship between the stationary elements, said second stationary element being provided with magnetic strips which are aligned with the magnetic strips on the other stationary element and are of opposite magnetic polarity to provide a magnetic field which extends essentially transverse to the movable element between aligned magnetic strips.
 5. The linear motor relay as claimed in claim 4 wherein said electrical winding is located on opposite planar surfaces as the movable element with the turns on one surface overlying spaces between turns on the other surface for enhanced interaction with the magnetic field between aligned magnetic strips.
 6. The linear motor relay as claimed in claim 1 including a plurality of movable elements coupled to one another to form a multilayered armature structure and a plurality of stationary elements coupled to one another, said plurality of elements being interleaved to form alternate layers of movable and stationary elements arranged in an electromagnetic drive structure capable of producing a substantial force upon the application of current to the electrical winding.
 7. A linear motor relay switch comprising a plurality of switches each having a stationary contact and a movable contact, a multi-layered structure formed of interleaved parallel mounted sets of stationary and movable plates, the set of movable plates being connected together to form a unified armature which moves along a linear direction, said unified armature being coupled to the movable contacts for controlling the opening and closing of the switches, means mounted to one set of plates for producing a magnetic field extending generally transverse to and between the plate surfaces along spaced longitudinal segments which are transverse to the direction of motion of the unified armature, and an electrical winding mounted to the other set of plates, said electrical winding being formed of groups of turns parallel to and in the path of the longitudinal segments of magnetic field and oriented to produce a common force on the unified armature when a current is passed through the electrical winding, and means for limiting the motion of the unified armature to maintain each group of turns in coacting relationship with a longitudinal segment of magnetic field for bidirectional control of the unified armature to open and close said switches.
 8. The linear motor relay switch as claimed in claim 7 wherein the magnetic field producing means includes strips of magnetic material of opposite magnetic polarity, said strips being mounted on the surfaces of said one set of plates and transverse to the linear direction, with magnetic strips on adjacent plates of the one set being of opposite polarity to establish said strip of magnetic field between them and through an interleaved plate of the other set.
 9. The linear motor relay switch as claim in claim 8 wherein the strips of magnetic material are mounted on the stationary plates and the electrical winding is mounted on the movable plates.
 10. The linear motor relay switch as claim in claim 9 wherein the groups of turns are distributed on opposite surfaces of a movable plate and are located to overlay spaces between adjacent turns on one surface.
 11. A linear motor relay comprising an electromagnetic structure formed of a generally planar stationary element and a generally planar movable element spaced from and mounted parallel with the stationary element for movement along an operating direction, both said stationary and movable elements being provided with planar windings formed of groups of turns oriented transverse to the operating direction and spaced uniformly from one another along the operating direction, with each group of turns on the movable element being in electromagnetic coacting relationship with corresponding groups of turns on the stationary element to produce a force on the movable element acting parallel to the operating direction upon the selective flow of current through the respective windings of the stationary and movable elements, and means for limiting the motion of the movable element along the operating direction to maintain eAch group of turns on the movable element in electromagnetic coacting relationship with its corresponding groups of turns on the stationary element for bidirectional control of the movable element.
 12. The linear motor relay as claimed in claim 11 and further including a multilayered structure formed of at least a pair of stationary planar elements and a movable planar element mounted between the pair of stationary elements.
 13. The linear motor relay as claimed in claim 12 and further including groups of turns located on opposite sides of the planar movable element with the groups of turns on each side mounted for electromagnetic coaction with a group of turns on the stationary elements.
 14. The linear motor relay as claimed in claim 13 and further including parallel strips of unmagnetized highly permeable material, said strips being located between groups of turns on both the movable and stationary elements.
 15. The linear motor relay as claimed in claim 11 wherein said motion limiting means further includes a linear ball bearing structure mounted to the movable and stationary elements, said structure having outer linear races mounted to the stationary element and inner linear races mounted to the movable element and bearing balls mounted for movement between opposed pairs of outer and inner linear races, said races being aligned along the operating direction with one of the races in each pair having a bulge sized to define stable bearing ball positions corresponding with the limits on the movement of the movable element. 